In wireless data communication, a method is being employed of using likelihood to estimate a correct signal from a signal corrupted by noise. Encoding in the method is a process of symbol-mapping data, i.e., a payload to be transmitted, into a larger number of signals to modulate the data at a transmitting side of a data communication system. And, decoding in the method is a process of estimating a payload having the highest likelihood from the symbol-mapped signals according to an appropriate estimation algorithm at a receiving side of the data communication system. In the encoding process using likelihood, it is preferable for the sake of accurate error correction that data is not just simply symbol-mapped based on its quantity but rather symbol-mapped to large areas of the frequency domain and the time domain. The above-described encoding-decoding method is used for data communication in fields where no data corruption can be tolerated and also is used to transmit a signal requiring a high degree of accuracy, such as a control signal, e.g., an ACK/NACK signal and a feedback signal, in general wireless data communication.
Meanwhile, various techniques of modulating amplitude or a frequency have been suggested as a method of transmitting data on a carrier. Among the techniques, Quadrature phase shift keying (QPSK) modulation changes the phase of a carrier by 90 degrees and transfers 2 bits of information using signs of one period. QPSK modulation enables accurate demodulation. Therefore, QPSK modulation is used in mobile communication equipment such as a digital cellular phone, a car phone, a digital cordless phone, etc., and is also used for a signal transmission scheme of recently provided wireless portable Internet service.
Looking back at the development of wireless data communication systems, cellular mobile telecommunication systems were first introduced in the United States in the late 1970's. This was followed by Korea's advanced mobile phone service (AMPS), an analog mode of the first generation (1G) mobile communication system enabling wireless voice communication. In the mid 1990's, the second generation (2G) mobile communication system was commercialized. This was followed in the late 1990's by commercialization of a part of the International Mobile Telecommunication-2000 (IMT-2000) standard, which has served as the third generation (3G) mobile communication system for providing high-speed wireless multimedia data service.
Nowadays, research is aimed at upgrading the 3G mobile communication system into the fourth generation (4G) mobile communication system. In particular, portable Internet technology is being vigorously researched with the goal of enabling faster data transmission than in a 3G mobile communication system.
The portable Internet satisfies users' demands for high-speed Internet service, anytime, anywhere, via a portable device, and has a ripple effect on the entire information and communication industry in Korea. Therefore, the portable Internet is a new and promising industry, and international standardization of the portable Internet is currently in progress on the basis of Institute of Electrical and Electronics Engineers (IEEE) 802.16e.
FIG. 1 schematically illustrates the structure of a portable Internet service system to which the present invention is applied. The illustrated portable Internet service system comprises portable subscriber stations (PSSs) 12, radio access stations (RASs) 13, access control routers (ACRs) 14, and an AAA (authentication authorization and accounting (AAA)) server 15. The PSSs 12 are used by subscribers to receive portable Internet service. The RASs 13 are located at wire network ends for transmitting and receiving data to and from the PSSs 12 through wireless interfaces. The ACRs 14 are for controlling the RASs 13 and routing Internet protocol (IP) packets. The policy (AAA) server 15 performs authentication, authorization and billing for a subscriber and a PSS 12, and provides service only to legal subscribers connected with the portable Internet network.
The PSSs 12 and the RASs 13 communicate using an Orthogonal Frequency Division Multiple Access (OFDMA) scheme. The OFDMA scheme is a multiplexing method combining a frequency division method (FDM), which uses subcarriers of a plurality of orthogonal frequencies as a plurality of subchannels, with a time division method (TDM). Since the OFDMA scheme is essentially robust against fading generated in a multi-path and has a high data transfer rate, it is possible to obtain optimum transfer efficiency in high-speed data transfer. Thus, the OFDMA scheme fully supports PSS mobility in portable Internet systems.
As described above, in order to ensure accuracy in transmitting and receiving an essential signal like a control signal, e.g., a fast feedback signal and an ACK/NACK signal, a wireless communication system based on the OFDMA scheme, etc. employs a modulation/encoding method whereby a payload is symbol-mapped in a sufficiently wide band channel and transmitted.
However, when a receiving side estimates a payload that has been symbol-mapped in a wireless channel as mentioned above, a received signal's likelihood with respect to a channel signal for all candidate payload values must be calculated, which becomes a heavy burden on the system of the receiving side.